Effects of heat stress on the metabolic transcriptional profile of peripheral tissues in growing pigs
Heat stress (HS) alters postabsorptive metabolism and nutrient partitioning, independently of reduced nutrient intake. Surprisingly, despite marked hypophagia, heat-stressed animals have reduced plasma non-esterified fatty acids (NEFA), and decreased sensitivity to lipolytic signals. In addition, HS increases plasma insulin parameters in a variety of animal models. Further, HS seems to alter systemic fuel utilization, favoring aerobic glycolysis rather than oxidative phosphorylation. Study objective was to determine if these metabolic changes have their origin at the transcriptional level. Seventeen crossbred gilts (57±5 kg BW) were subjected to one of two environmental treatments: 1) constant HS conditions (32°C, 23% RH) and ad libitum feeding (n=7), or 2) pair-feeding in thermoneutral conditions (20°C, 36% RH; PFTN; n=10) to eliminate the confounding effects of dissimilar intake. Feed intake decreased 38% on average and was not different between treatments (P = 0.75). After 8d of environmental exposure, pigs were sacrificed and liver, subcutaneous adipose tissue (AT) and muscle (longissimus dorsi, LD) immediately collected. Gene expression was determined using qPCR (BioMarkTM System, Fluidigm Corporation) on an average of 42 genes per tissue. Genes were selected based on the RNA-Seq output of a similar experiment. Gene expression was normalized to housekeeper genes and statistical analysis was performed in delta delta Ct values (PROC GLM, SAS 9.2). Data is reported as fold change. As expected, heat shock protein-related genes (e.g. HSF2, HSPA4, HSPB8, HSPCB, HSPE1, HSP90AA1) were up-regulated (41-156%, P ≤ 0.10) across all tissues in HS compared to PFTN conditions. Supporting the phenotypic observation, adipose triglyceride lipase was down-regulated (36%, P = 0.07), hepatic fatty acid synthase was up-regulated (1.5 fold, P < 0.01), and TCA cycle and electron transport chain proteins (i.e. IDH2, NDUFB7, NDUFS7) were down-regulated (43%, P = 0.06; 22%, P = 0.05; 23%, P = 0.10; respectively) in liver and LD of HS pigs. Unexpectedly, AT insulin receptor and LD pyruvate dehydrogenase kinase 4 were down-regulated (32%, P = 0.07; 61%, P = 0.05; respectively) in HS compared to PFTN pigs. Abundance of most of the genes involved in bioenergetic pathways did not differ between treatments. These data suggest that changes in metabolism and fuel selection after chronic HS (8d) may partially arise from differences in post-transcriptional regulation. Whether gene expression control at the transcriptional level has a role on metabolic adaptation to acute HS remains unknown.
Keywords: Heat stress, pig, postabsorptive metabolism